In your case, probably nothing. If you had a vinyl lined pool, water might get down between the liner and the wall. If you have an above-ground pool, there will always be a low spot, even if it's only 1/16" and that's where the water will drain. That area will end up getting softer than the rest, which may make the pool sink on that side, which makes it drain more, and eventually the pool is visibly off level and might collapse.
I like to drain in advance because I divert the rain gutters to my pool to reduce the CH buildup. If my water is 2" too high, I'd rather lose 2" of saturated water than 2" of diluted water.